The present invention relates a flexible waveguide tube. Specifically, the invention relates to a flexible waveguide tube for connection of a waveguide circuit having sufficient strength to be used for connection between on-board equipment in a satellite.
In general, the dimensions of a rectangular waveguide tube for a millimeter wave band are quite small, such as 5.7 mm in its longitudinal dimension and 2.85 mm in its transverse dimension at the 40 GHz band. Therefore, it is quite difficult to produce a flexible waveguide tube with a sufficient strength for such wave band. In particular, in case of the waveguide tube connection circuit to be mounted on a satellite, it is required to have sufficient strength for withstanding the severe vibrations that accompany the launching of the satellite. Therefore, such flexible waveguide tube is required to withstand severe vibrations of 19.6 grms. The rectangular waveguide tube produced to provide the flexible waveguide tube with a sufficient strength is 7.1 mm in longitudinal dimension and 3.5 mm in its transverse dimension. This limits the frequency band that may be used to between 26.5 to 40 GHz.
FIG. 11 shows an external appearance of the conventional flexible waveguide in an assembled condition. Also a cross-sectional view of the flexible waveguide along the center line of the longer diameter is shown in FIG. 12.
As shown in FIG. 11, the conventional flexible waveguide tube includes rectangular tube portions 2 at both ends of a bellows portion 1. Flanges 5 are further provided for connection with other waveguide tubes, which are not shown. Since the bellows portion 1 is provided, the waveguide can be bent in a direction shown by an arrow Y1 in FIG. 11. The flexible waveguide tube can also be bent in the direction Y2, also shown in FIG. 11. It should be noted that the reference numeral 6 denotes a mounting holes.
With reference to FIG. 12, the cross-section of the walls of bellows portion 1 are wavy in configuration, and this wavy configuration has an amplitude H of 0.5 mm.
Since the excessive amplitude of the wavy wall could influence the characteristics of the waveguide, the amplitude H should be as small as possible. However, in view of the current technology in processing, it is difficult to make the amplitude smaller than approximately 0.5 mm.
The assembled flexible waveguide tube was evaluated relative to transmission loss versus frequency. The results of this evaluation is shown in FIG. 13. In FIG. 13, the transmission loss was 1.5 dB and a transmission loss difference (difference between a peak value and a minimum value) in the 200 MHz band width was 1.3 dB. However, this performance cannot satisfy a required performance of less than or equal to 0.5 dB in the transmission loss and 0.2 dB in transmission loss difference.
The above-mentioned conventional flexible waveguide tube has large transmission losses and the transmission loss difference in the millimeter wave band is higher than or equal to 40 GHz, and thus it cannot be used as the waveguide connection circuit installed in a satellite.
On the other hand, Japanese Unexamined Patent Publication No. 60-180302 discloses a tapered waveguide tube for connecting two circular waveguide tubes having mutually different diameters. The principle of the above-identified prior art is as follows. Since the waveguide tubes having mutually different diameters, they cannot be directly connected because of differences in impedances. Therefore, in order to match the impedances, connection is established by means of the tapered waveguide tube, and the interior of the waveguide tube is filled with a bar-shaped dielectric body.
Such construction is effective in connection of the waveguides having mutually different diameters with matching of the impedances. However, since it takes the construction completely filled with the bar-shaped dielectric body, it cannot provide flexibility when the above-mentioned construction is employed as the flexible waveguide tube.